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From: ggs@ulysses.homer.nj.att.com (Griff Smith)
Newsgroups: comp.lang.c
Subject: constant expression rules
Message-ID: <3018@ulysses.homer.nj.att.com>
Date: Fri, 25-Sep-87 11:02:12 EDT
Article-I.D.: ulysses.3018
Posted: Fri Sep 25 11:02:12 1987
Date-Received: Sun, 27-Sep-87 02:50:55 EDT
Organization: AT&T Bell Laboratories, Murray Hill
Lines: 44
Keywords: portability

A user of our system grumbled recently that our C compiler is broken.
He tried to compile the following on our 4.3BSD CCI POWER 6/32 C
compiler:

int j = ((int )(.5 * 4));

The response from the compiler was

"foo.c", line 1: illegal initialization

I then wrapped a function around it and compiled it on a 4.3BSD VAX
and on the 4.3BSD tahoe compiler.  The VAX produced the following
assembly language fragment:

	movl	$2,-4(fp)	# 0.5 * 4 has been reduced to 2
	movl	-4(fp),r0
	ret

The tahoe did it the hard way:

	.data
	.align	2
L20:
	.long	0x41000000, 0x00000000 # .double 2
	.text
	ldd	L20		# note that .5 * 4 is reduced to 2.0
	cvdl	r0		# convert to long the hard way
	movl	r0,-56(fp)
	movl	-56(fp),r0
	ret#1

I think this explains the problem with constant initialization on the
tahoe compiler; the expression is "too complex".  I also tried the
4.2BSD VAX C compiler and got output similar to the tahoe.

The question is - what is a reasonable level of constant folding that
I can assume when trying to write portable code?  Is this a bug or a
gotcha?  K&R says that I can have almost any set of constants connected
by +, -, *, ?: and a host of others (page 211).
-- 
Griff Smith	AT&T (Bell Laboratories), Murray Hill
Phone:		1-201-582-7736
UUCP:		{allegra|ihnp4}!ulysses!ggs
Internet:	ggs@ulysses.uucp